In the manufacture of metal-containing components which are to be soldered, it is helpful to protect the metal from oxidation to enhance its solderability. For convenience, the following description is directed to the manufacture of printed wiring boards (PWB) or other electronic components having copper circuitry which is to be soldered and where it is desired that the copper not be oxidized or otherwise corroded prior to soldering.
The processes used today for protecting copper prior to soldering employ a protective coating deposited on the copper. This coating is used as an alternative to hot air solder leveling (HASL) and other metallic printed circuit board surface finishes. The coating provides protection against copper solderability degradation caused by various process steps in the fabrication process such as exposure to multiple heat cycles during electronic component fabrication for surface mount technology (SMT) and mixed technology PWB assembly.
Generally, copper protective coating systems utilize a number of steps including cleaning, microetching, and acid rinsing followed by the formation of a protective coating on the copper using a solution containing a protective forming agent. The protective forming agent is typically an imidazole or benzimidazole derivative, and the coating is generally termed an organic solderability preservative (OSP) coating.
A number of patents have issued in this area for processes which have attempted to solve the problems associated with providing a protective coating on copper circuitry.
U.S. Pat. No. 5,658,611 to Ishiko et al. provides an aqueous surface protection composition for PWBs containing a benzimidazole derivative and adjusted to a pH of 1-5 with a salt forming acid of a heavy metal such as copper, manganese, and zinc in an amount not higher than 50 ppm.
In U.S. Pat. No. 5,173,130 to Kinoshita et al., a process is disclosed for the surface treatment of copper which comprises immersing the surface of the copper in an aqueous solution containing a benzimidazole compound having an alkyl group of at least three carbon atoms at the 2-position and an organic acid. Similarly, in U.S. Pat. Nos. 5,498,301 and 5,560,785 to Hirao et al., a water-based surface treatment agent used to protect copper on a printed wiring board with excellent heat-resistance and moisture-resistance utilizes as an active ingredient a 2-arylimidazole compound.
In U.S. Pat. No. 5,795,409 to Hirao et al., a surface treating agent for copper is disclosed comprising an aqueous solution containing an imidazole compound or a benzimidazole compound, a complexing agent, and iron ions. It is contended that the surface treating agent forms a chemical film selectively on the surface of copper while forming no film on other metals.
In U.S. Pat. No. 5,362,334 to Adams et al., a composition and process for the surface treatment of metallic surfaces such as copper circuitry on printed circuit boards is disclosed which comprises treating the surface with an aqueous solution comprising a benzimidazole compound having either a halogenated phenyl group, a halogenated benzyl group, or a halogenated ethyl phenyl group in the 2-position.
In U.S. Pat. No. 5,376,189 to Kukanskis, a composition and process are disclosed for the treatment of metallic surfaces such as copper on printed circuit boards which comprises treating the surface with an aqueous solution comprising a benzimidazole compound which has at least one carboxylic or sulfonic acid group directly or indirectly attached to the benzimidazole compound.
A problem shared by many of these aqueous-based OSP compositions is related to the solubility of the azole compound. It has been observed that many of the azole compounds in the above-described patents, such as those comprising bulky 2-alkyl and 2-aryl substituents have low miscibility with water, which presents a problem in the preparation of the composition. In some preparation processes, lower alkyl alcohols, for example, alcohols having three or fewer carbon atoms and characterized by high volatility and low boiling point, have been employed to solubilize the azole compounds during OSP composition makeup. However, over time, many of the azole compounds crystallize. It is thought that the lower alkyl alcohols evaporate over time, rendering the azole compounds less soluble in the OSP composition. The OSP compositions become less concentrated in the azole compound, which compromises the ability of the OSP composition to form an effective protective coating over a copper substrate. Moreover, OSP compositions in which azole compounds have become sparingly soluble tend to deposit crystals on treated PWBs and on the tool parts which transfer PCBs through the production line.